1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to a signal transmission method and apparatus for a mobile communication system using a multiple antenna.
2. Description of the Background Art
Recently, with the rapid growth in the radio mobile communication markets, diverse multimedia services are in a great demand in the radio environment, and especially, as a transmit data is growingly increased and a data transmission speed becomes fast. Therefore, finding a way to efficiently use a limited frequency is the most critical task. In an effort to cope with the subject, a fresh transmission technique using multiple antennas is required for use, and one example of which is a multiple input multiple output (MIMO) system using the multiple antenna.
FIG. 1 illustrates one example of a general mobile communication system adopting a MIMO system.
As shown in FIG. 1, a conventional MIMO system includes: a plurality of transmitting antennas 12, a vector encoder 10 for transmitting sequentially generated transmit data (symbols) to each transmitting antenna 12; a plurality of receiving antennas 14; and a V-BLAST signal processor 16 for processing data received through the receiving antennas 14 in a V-BLAST (Vertical-Bell Labs Layered Space Time Architecture) method, and estimating and demodulating a transmit data.
The vector encoder 10 converts the sequentially generated transmit data in a serial-to-parallel method and transmits them to each transmitting antenna 12.
The V-BLAST METHOD is a sort of a transmission technique of a MIMO mobile communication system using multiple antennas, for which M number of transmitting antennas 12 and N number of receiving antennas 14 are used. FIG. 1 shows the case of using 4 transmitting antennas 12, but without being limited thereto, two or more antennas can be set arbitrarily.
The signal processing operation of the general MIMO mobile communication system constructed as described above will now be explained.
Without performing a certain signal processing for enhancing a transmission quality on the transmit data, the vector encoder 10 simply processes the transmit data a1-a4 in parallel and transmits them to each antenna 12. Then, each transmitting antenna 12 transmits each different transmit data in the air.
Data received through each receiving antenna 14 is inputted to the V-BLAST signal processor 16, and the V-BLAST signal processor 16 performs a signal processing suitably, that is, in a V-BLAST method, and detects the transmit data a1-a4.
The operation of the V-BLAST signal processor 16 will now be described in detail.
When the transmitting antenna 12 of the transmitting antenna array transmit a different transmit data (transmit symbol) to each other, the V-BLAST signal processor 16 receives the reception signals from each receiving antenna 14 of the receiving antenna array and constructs a reception data vector (receive vector). Subsequently, the V-BLAST signal processor 16 detects a transmission signal by using the receive vector.
In this respect, while the V-BLAST signal processor 16 detects the transmit symbol a specific transmitting antenna has transmitted, it regards transmit symbols other transmitting antennas 12 have transmitted as an interference signal.
Thus, the V-BLAST signal processor 16 calculates a weight vector of each receiving antenna 14 for each symbol transmitted from each transmitting antenna 12 and simultaneously subtracts the first detected symbol component from the receive vector, thereby estimating each symbol while minimizing influence of each symbol.
FIG. 2 is a flow chart of a method for estimating the transmit symbol by the V-BLAST signal processor 16.
First, the V-BLAST signal processor 16 constructs the signals received through each receiving antenna 14 as receive vectors (step S11).
For example, in case of a MIMO mobile communication system having M number of transmitting antennas 12 and N number of receiving antennas 14, assuming that a signal vector (transmit vector) transmitted through the M number of transmitting antennas 12 is ‘a’ and a matrix of a mobile communication channel (channel matrix) through which the transmit vector is transmitted is ‘H’, the receive vector (R) can be expressed by the following equation (1):R=H×a+v  (1)
At this time, since the signals transmitted from the M number of transmitting antennas 12 are received through the N number of receiving antennas 16 through a different path, the channel matrix (H) can be expressed by N×M matrix. The channel matrix (H) is obtained through estimation by the V-BLAST signal processor 18. ‘v’ in equation (1) is a Gaussian noise, Since the noise is induced to each receiving antenna 14, ‘v’ is N×1 vector.
Consequently, the signals transmitted through the M number of transmitting antennas 12 pass through a different communication channel (hi,j), and the V-BLAST signal processor 16 receives the signals through the N number of receiving antennas 14.
Upon receiving them, the V-BLAST signal processor 16 calculates each weight vector of the signals, and estimates symbols transmitted from each transmitting antenna 12 by using the calculated weight vector and the receive vector.
First, a method for calculating the weight vector will now be described.
In order for a receiving end to detect the symbols transmitted from the M number of transmitting antennas 12, signals received by the N number of receiving antennas are inner-producted by a weight vector which is defined as ‘w’. Since different symbols are transmitted through the M number of transmitting antennas 12, the M number of weight vectors are required for the V-BLAST signal processor 18 to detect the transmit symbols. At this time, the weight vector (w) should satisfy the following condition.
                                          w            i            H                    ×                      H            j                          =                  {                                                                      0                  ⁢                                      (                                          j                      ≥                      i                                        )                                                                                                                        1                  ⁢                                      (                                          j                      =                      i                                        )                                                                                                          (        2        )            wherein ‘Hj indicates a vector in the jth column of the channel matrix (H) which can be estimated by the V-BLAST signal processor 18.
In equation (2), the weight vector (wi), which should be inner-producted to a corresponding receive vector so as for the ith transmitting antenna to detect a symbol transmitted from the antenna, has a property that it is ‘1’ only when inner-producted to the ith column vector of the channel matrix (H) and ‘0’ when inner-producted to the other remaining column vectors of the channel matrix (H).
That is, in the case of the weight vector (wi) for detecting the ith transmit symbol, influence of symbols transmitted through other transmitting antennas should be removed.
In addition, the transmit symbols are sequentially detected, and when the weight vector to be used for detecting a current symbol is obtained, since any influence of the previously detected symbols should be excluded, the expression ‘j≧l’ is used in equation (2).
Thus, the weight vector satisfying the quality of equation (2) can be obtained as follows: To begin with, the receive vector of equation (1) can be expressed by the following equation (3):R=a1H1+a2H2+ . . . +aMHM  (3)
In general, symbols transmitted from each transmitting antenna 12 are received by the receiving end through each different channel, and equation (3) expresses the receive vector with the received symbols as a shape of a linear sum.
As noted in equation (3), when the first transmit symbol is detected, it is preferred that influence of second to Mth symbols is removed and the weight vector is then inner-producted to the receive vector. The same principle can be applied to the case of detecting other transmit symbols.
When a specific transmit symbol is detected, in order for a corresponding weight vector not to be influenced from other transmit symbols, the V-BLAST signal processor 16 updates the weight vector for every transmit symbol to be detected and uses it.
Once the receive vector (R) is constructed and the channel matrix (H) is estimated, the V-BLAST signal processor 16 starts updating a weight vector in order to obtain a weight vector for each transmit symbol to be detected.
For this purpose, as noted in the below equation (4), the V-BLAST signal processor 16 obtains a moore-penrose pseudoinverse matrix (H+ or G1) for the estimated channel matrix (H) (step S13).G1=H+  (4)
After obtaining the Moore-Penrose pseudoinverse matrix, the V-BLAST signal processor 16 selects a row vector with the smallest vector norm value from row vectors of a G1 matrix, as a weight vector (step S15).
For instance, on the assumption that the selected row vector is the Kth row vector, the Kth line of the G1 matrix is selected as a weight vector (wK) for detecting the Kth transmit symbol.
After the weight vector (wK) is selected, the V-BLAST signal processor 16 inner-products the receive vector (r) and the weight vector (wK) to estimate a symbol transmitted from the Kth transmitting antenna (step S17).
The receiving end, that is, the V-BLAST signal processor 16, is well aware of a modulation method (i.e., QPSK, QAM, etc.) used in a sending end of the MIMO mobile communication system. Thus, as the V-BLAST signal processor 18 is able to recognize which constellation the estimated symbol belongs to, it determines the estimated symbol as a transmit symbol (ak) transmitted form the Kth transmitting antenna.
The V-BLAST signal processor 18 checks whether the M number of transmit symbols transmitted from the M number of transmitting antennas 12 have been all detected (step S21). If there still remains transmit symbols to be detected, the V-BLAST signal processor 18 performs a procedure for updating the weight vector in order to detect the remaining transmit symbols.
First, when Kth symbol (ak) is detected, as shown in the below equation (5), the V-BLAST signal processor 18 removes influence of the Kth symbol (aKHK) from the receive vector (r) of equation (3) to obtain a receive vector (r2) to be used for the second updating (step S23).r2=r−aKHK  (5)
After the receive vector (r1) is obtained, the V-BALST signal processor 18 estimates a channel matrix (H2) to be used to obtain G2, that is, the second weight vector (step S25). Namely, the V-BLAST signal processor 16 deletes the column vector (Kth column) corresponding to the detected transmit symbol (ak) from the previous channel matrix (H) and estimates a new channel matrix (H2).
Subsequently, advancing to the step (S13), the V-BLAST signal processor 16 calculates the Moore-Penrose pseudoinverse matrix of the estimated channel matrix (H2) as shown in the below equation (6):
                    G2        =                              H                          _              _                                            K            _                    +                                    (        6        )            
The V-BLAST signal processor 18 selects a row vector with the smallest vector norm from row vectors of the G2 matrix, as a weight vector (step S15). For instance, assumption that the selected row vector is the Vth row vector, the Vth line of the G2 matrix is used as a weight vector (wV) for detecting the Vth transmit symbol.
After the weight vector (wV) is selected, the V-BLAST signal processor 16 inner-products the receive vector (r2) and the weight vector (wV) to estimate a symbol transmitted from the Vth transmitting antenna (step S17).
As stated above, the V-BLAST signal processor 18 is already aware of the digital modulation method used in the sending end of the MIMO mobile communication system, it can judge which constellation the estimated symbol belongs to, based on which the V-BLAST signal processor detects a symbol (aV) transmitted from the Vth transmitting antenna. (step S19).
Thereafter, the V-BLAST signal processor 18 checks whether the M number of transmit symbols transmitted from the M number of transmitting antennas 12 have been all detected (step S21). If not all transmit symbols have been detected, the operations after the steps S23 and S25 are repeatedly performed. If all the transmit symbols have been detected, the steps are terminated.
As stated above, in the general MIMO mobile communication system, the transmit symbol is simply converted serial-to-parallel without being subjected to an additional signal processing, and then transmitted through the transmitting antennas. Then, the receiving end sequentially detects the transmit data each transmitting antenna has transmitted. At this time, the symbols transmitted independently from each transmitting antenna should maintain their independence while passing through the mobile communication channel.
However, actually some degree of correlation exists between the respective Tx antennas of the Tx antenna array and the correlation exists between the Rx antennas of the Rx antenna array, and therefore, the independence of the signals transmitted from the respective Tx antennas can not be ensured. Also, the mobile communication channel should ensure the independent channels between the respective Tx antennas and the respective Rx antennas, however, may not ensure the independent channels as many as the number of respective Tx antennas actually.
Therefore, the receiving end of the general MIMO mobile communication system has difficulty to detect the signal transmitted from a certain Tx antenna independently, and accordingly, an error may be generated and function of signal detecting is lowered, and thereby, the system function may be lowered.